The present invention relates to an active matrix liquid crystal display device comprising a row and column array of liquid crystal display elements, each display element comprising a display element electrode connected to an associated switching device, sets of row and column address conductors connected to the display elements and via which selection signals and data signals respectively are applied to the display elements, a row drive circuit for applying selection signals to the set of row address conductors, and a column drive circuit for applying data signals to the set of column address conductors via respective outputs, which column drive circuit is operable such that an output thereof associated with one column address conductor becomes high impedance prior to or while the data signal for an adjacent column address conductor is applied.
Active matrix liquid crystal display devices suitable for displaying datagraphic or video information are well known. Typical examples of such, and the general manner in which they operate, are described in U.S. Pat. No. 5,130,829. In these devices, the display element electrodes, organised in rows and columns, are provided on a first substrate together with the switching devices, in the form of TFTs (thin film transistors), and the sets of row and column address conductors. A second substrate carrying a transparent common electrode is arranged spaced from the first substrate and LC (liquid crystal) material is disposed between the two substrates, with each display element being defined by a respective display element electrode and the overlying portion of the common electrode together with the LC material therebetween. Each display element electrode is connected to the drain electrode of its associated TFT. The TFT of each display element is connected to respective ones of the row and column address conductors with the gates of all the TFTs in a row of display elements being connected to a respective row address conductor and the source electrodes of all the TFTs in a column of display elements being connected to a respective column address conductor. Each display element electrode is situated adjacent the intersection of its associated row and column conductors, which extend along two adjoining sides of the electrode. Adjacent row and column conductors extend along the other sides of the electrode so that each display element electrode is bordered by adjacent pairs of row conductors and column conductors. A row drive circuit connected to the set of row address conductors scans the row conductors and applies a selection (gating) signal to each row conductor in sequence to turn on the TFTs of a row of display elements and a column drive circuit connected to the set of column conductors applies data signals to the column conductors in synchronism with scanning of the row conductors by the row drive circuit whereby the display elements of a selected row are charged via their respective TFTs to a level dependent on the value of the data signal on their associated column conductors to produce a required display output. The rows are driven individually in turn during respective row address periods in this manner so as to build up a display picture over one field period, and the array of display elements is repeatedly addressed in similar manner in successive field periods.
For convenience of manufacture and compactness, the row and/or column drive circuits in some display devices, and particularly those using polysilicon TFTs, have been integrated on the substrate carrying the TFTs peripherally of the display element array using the same large area electronics technology as that employed for the active matrix circuitry of the display element array with the circuitry of the drive circuits being fabricated simultaneously with the active matrix circuitry and similarly comprising TFTs, conductor lines, etc. This avoids the need to use separately manufactured drive circuits that need to be interconnected to the address conductors of the display element array on the substrate. Due to limitations in the performance of the TFTs and the kinds of circuit possible when using TFTs, the column drive circuit is customarily provided in the form of a simple multiplexing circuit, examples are which are described in the paper entitled xe2x80x9cFully Integrated Poly-Si TFT CMOS Drivers for Self-Scanned Light Valvexe2x80x9d by Y. Nishihara et al in SID 92 Digest, pages 609-612, and in the paper entitled xe2x80x9cA 1.8-in Poly-Si TFTxe2x80x94LCD for HDTV Projectors with a 5-V Fully Integrated Driverxe2x80x9d by S. Higashi et al in SID 95 Digest, pages 81 to 84. Such a circuit operates in the manner described in the opening paragraph. Their general operation is based on a multiplexing technique in which analogue video information (data) is sequentially transferred via multiplexing switches from video input lines to corresponding groups or blocks of column address conductors in the display. The video information is applied simultaneously to a number of video input lines and transferred via the multiplexing switches to a corresponding number of column address conductors. During a row address (video line) period each group of column conductors is charged in turn until all the column conductors in the display device have been charged to a level corresponding to the level of the video information on the input lines. Once a group of column conductors has been charged the associated multiplexing switches open and the column conductors become high impedance nodes with the voltage applied being maintained on the column conductor capacitance and the next group is charged. The circuit operates in this manner so as to charge all the groups in sequence and to drive each row of display elements in turn in this way during respective row address periods.
Whilst the provision of an integrated, multiplexing type, column drive circuit has benefits with regard to the simplication of fabrication of the display device, it has been found that problems can occur in the display output from the display element array during operation of the device. Certain columns in the array show errors in their display brightness, for example a lack of display uniformity when displaying grey fields which manifests itself as highly visible vertical lines in the displayed image.
It is an object of the present invention to provide an active matrix display device of the kind using a column drive circuit which operates in the manner of a multiplexing circuit in which the problem of the aforementioned undesirable display output artifacts is overcome or reduced at least to some extent.
According to the present invention there is provided an active matrix liquid crystal display device of the kind described in the opening paragraph which is characterised in that the column address conductor associated with a display element is arranged to lie inwardly of the display element electrode edges. Preferably, the column address conductor is positioned towards the middle of the display element electrode. As a result of arranging the column address conductors in this way, it has been found that the extent of unwanted display artefacts in the form of visible vertical lines is at least considerably reduced.
The invention stems from an appreciation of the reason for these display artefacts when using a multiplexing type of column drive circuit. In the conventional display element lay-out, a column address conductor associated with a particular display element extends alongside one vertical edge, or side, of the display element electrode and another column address conductor, associated with the adjacent column of display elements, extends alongside the opposing vertical edge. Thus, each column conductor extends between an adjacent pair of display element electrodes in a row and alongside the facing edges of the electrodes. The capacitance coupling between an adjacent pair of column address conductors indirectly via the electrode can therefore be significant. Direct capacitive coupling between two column conductors can occur in the case of an alternative lay-out in which pairs of column conductors are provided adjacent one another and columns of display element electrodes are provided to either side of the pair, one column of electrodes being addressed via one of column conductors and the other addressed via the second column conductor. The presence of such indirect or direct capacitance means that as the voltage on the first column conductor of one group is charged in operation of the column drive circuit the change in voltage can be coupled onto the last column conductor of the previously addressed group through this capacitance, thereby disturbing the voltage set on that last column conductor. The result is that errors occur in the voltage on the last column conductor of each group which errors cause the visible vertical lines in the displayed image. The problem is particularly apparent in high aperture type display devices, for example of the kind described in U.S. Pat. No. 564,194 and EP-A-0617310, in which the display element electrodes are carried on an insulating layer that extends over the active matrix circuitry, comprising the TFTs and sets of row and column address conductors, on the substrate and in which portions of the display element electrodes are arranged to overlap the two adjacent column address conductor (and row address conductors) so as to increase their effective apertures. Such overlap can result in significant capacitance existing between a column address conductor and the adjacent portions of the display element electrodes. By arranging the column address conductors in relation to the display element electrodes in accordance with the present invention the extent of capacitive couplings between adjacent column address conductors is considerably reduced. In these high aperture kinds of display devices, the column conductors can readily be arranged instead beneath the display element electrode and inwardly of its edges, for example substantially centrally, rather than close to the electrode edges as the electrodes are carried an insulating layer at a different level to the active matrix circuitry.
In the case of a display device operating in transmissive mode and using transparent conductive material such as ITO for the display element electrodes, then the provision of the column conductors inwardly of the electrode will reduce the effective pixel aperture slightly when the conductors are formed of a light opaque material such as a metal rather than a light transparent material. However, in the case of a reflective display device having light reflective display element electrodes, then disposing the conductors beneath the electrodes in this way does not affect the aperture.
When formed of metal, the column address conductors extending between adjacent columns of display element electrodes in the known display devices may serve also as light shields which together with metal row address conductors constitute a black matrix bordering the individual display elements for the purpose of enhancing display contrast. Because the column address conductors in the display device according to the present invention no longer occupy the gaps between the columns of display element electrodes it may be desirable to block these gaps so as to avoid the possibility of the display contrast being degraded. In a preferred embodiment, display element storage capacitor electrodes of light opaque material are utilised to mask these gaps. Other approaches such as using the row metallisation or other layers, e.g. black matrix on the other substrate, are also possible.
It is envisaged that the invention can be used beneficially in display devices using column drive circuits other than of the multiplexing type but which likewise operate in such a way that an output associated with one column conductor becomes high impedance before or while an adjacent column conductor is being supplied with a data signal as similar problems would be experienced.